test: added validation regex argument to test

In this change, I've added the regex pattern to the assert.throws()
in order to provide the validation argument for the call.

PR-URL: https://github.com/nodejs/node/pull/9918
Reviewed-By: Teddy Katz <teddy.katz@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/parallel/test-file-write-stream.js

@@ -46,7 +46,7 @@ file
     assert.throws(function() {
       console.error('write after end should not be allowed');
       file.write('should not work anymore');
-    });
+    }, /^Error: write after end$/);
 
     fs.unlinkSync(fn);
   });